AC/DC hybrid power system

[0001] This application claims priority of Provisional Application Ser. No. 60/424,678 filed Nov. 8, 2002 which is herein incorporated by reference.

[0002] Solar power has been used for electric generation in countries where an AC grid power was not available. With the changes occurring in power generation, the cost of electricity, and energy rebates, solar power becomes more attractive, even in developed countries with AC grid power. However, solar power is dependent on sun conditions and is therefore a variable power source. It is not available at nighttime or on cloudy days.

[0003] Accordingly, a hybrid system that uses solar power in conjunction with an AC power grid would be very useful. By combining AC grid power with solar power, the useful operating hours of the system can be extended beyond the normal operation of the solar power component while the use of the AC grid power component can be minimized.

[0004] The invention is a hybrid solar—AC grid (or other) power system. A DC solar power source is the primary power source. A rectified AC voltage from an AC power grid (or other source) provides a secondary DC power source. These two DC sources—solar and rectifier AC grid (or other)—are combined through blocking diodes or other selection components to the DC input terminals of a variable speed drive. Either or both voltage sources can be selected, depending on the magnitude of the voltage from each source. The variable speed drive uses the DC input voltage to produce a single phase or 3-phase voltage to a motor or other load.

[0011] In the hybrid power system of the invention, a DC solar power source is chosen to be the primary source and rectified AC from an AC grid is the backup power source. The two power sources are selectably coupled together through blocking diodes or relays or other selection components to a variable speed drive so that a sufficient voltage is always provided to a load, e.g. motor. When the solar power source produces a sufficient voltage for the load, it provides the power to the load. When the solar power source provides an insufficient voltage, then the backup power source is selected to power the load. Thus the load always receives sufficient power and the AC grid is only used when the solar power source is insufficient.

[0012] A minimum solar voltage level is chosen such that below this level, solar is not effective to drive the load, causing the load (e.g. motor) to go into shutdown or possibly stalling. By selecting the AC rectified voltage and controlling its peak voltage (VACpeak) to be equal to or slightly greater than the minimum solar voltage, uninterrupted power control can be provided to the variable speed drive. During cloudy days, or late evening or nighttime, the backup rectified AC source becomes the primary source and will supply power. During sunny daytime hours, solar becomes the primary source. The switching between solar and rectified AC power is transparent since both sources are available at all times (to the extent that the solar system is producing any power). The switching between the two power supplies is controlled through selection components, typically blocking diodes or SCRs or relays, which pass one supply and block the other supply depending on the voltages produced by each supply. One selection component will be forward biased or on while the other will be reverse biased or off. FIG. 6 shows a hybrid system 25, generally as described below, where one source is a solar DC source (SOLAR VDC) and the other source is a rectified AC source (RECTIFIED VAC) connected through relays 26, 27.

[0013] If an AC motor is being operated with rectified AC power as the primary source, solar can be used in combination with the AC grid power even if the solar power is not adequate to power the motor by itself. A variable speed drive, which produces either a variable frequency or fixed frequency output, is used to drive the motor. The DC input voltage to the drive is set at a predetermined voltage level that permits the sharing of power between the two sources. When the solar voltage drops below this predetermined value, the rectified AC source becomes the primary source. When solar power is equal to the predetermined value, both sources are supplying power. Again, the switching is automatic and transparent to the variable speed drive when combined with blocking diodes. Thus the solar source supplements the AC grid as much as possible and reduces use of grid power.

[0014] A simple hybrid power system 10 of the invention, as shown in FIG. 1, has a pair of DC sources V1, V2 connected through respective blocking diodes D1, D2 to the DC input of a variable frequency drive 12 whose output is connected to a load 14. Load 14 is typically a motor and drive 12 produces a single phase or 3-phase variable frequency or fixed frequency AC output to drive load 14. The voltages V1, V2 may be variable, e.g. V2 may be a solar power source such as a photovoltaic panel whose output changes with changing sun conditions. The power requirements of load 14 may also change. By connecting the two voltage sources V1, V2 to drive 12 through blocking diodes D1, D2, the higher voltage source will always be connected to drive 12. When V1>V2, D1 is forward biased while D2 is back biased so V1 is the primary source. When V2>V1, D2 is forward biased while D1 is back biased so V2 is the primary source. Thus if V2 is a solar panel, and V1 is another DC source, e.g. battery or rectified AC voltage, power is obtained from the solar panel when V2>V1; otherwise power will be obtained from the other source. If there are temporary fluctuations when V2 drops below V1, then V1 automatically switches in and the load does not see any change in power. FIG. 5 shows a hybrid system 15 where one source is a solar DC source (SOLAR VDC) and the other source is a rectified AC source (RECTIFIED VAC) connected through blocking diodes D1, D2.

[0015] Another simple hybrid power system 20 of the invention, as shown in FIG. 2, has a DC source VDC and a single phase AC source VAC connected through respective blocking diodes D2, D1 to the DC input of a variable frequency drive 12 whose output is connected to a load 14. The voltages VDC, VAC may be variable, e.g. VDC may be a solar power source such as a photovoltaic panel whose output changes with changing sun conditions. By connecting the two voltage sources VAC, VDC to drive 12 through blocking diodes D1, D2, the higher voltage source will always be connected to drive 12. When VACpeak>VDC, D1 is forward biased while D2 is back biased so VAC is the primary source. When VDC>VACpeak, D2 is forward biased while D1 is back biased so VDC is the primary source. Thus if VDC is a solar panel, and VAC is an AC grid, the solar source VDC will power the load when it produces sufficient voltage; otherwise power will be obtained from the other source. If there are temporary fluctuations when VDC drops below VACpeak, then VAC automatically switches in and the load does not see any change in power.

[0016] Another simple hybrid power system 30 of the invention, as shown in FIG. 3, has a DC source VDC and a 3-phase AC source having three windings L1, L2, L3 connected through respective blocking diodes D2, and D1-1, D1-2, D1-3 to the DC input of a variable frequency drive 12 whose output is connected to a load 14. The system 30 is similar to system 20. When Vpeak of L1, L2, L3 is greater than VDC, the 3-phase AC supply is the primary supply; otherwise, VDC is the primary supply.

[0017] Hybrid power system 40, shown in FIG. 4, is similar to system 30 but the three blocking diodes D1-1, D1-2, D1-3 connected to the windings L1, L2, L3 of the 3-phase AC source are replaced by silicon controlled rectifiers SCR-1, SCR-2, SCR-3. This allows for variable DC voltage to be applied to the variable frequency motor drive.